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Queen bees lay eggs singly in cells of the honeycomb. After the eggs hatch, worker bees feed the larvae in the cells and cap them when the larvae pupate. A drone is pictured emerging from a cell.

Queen bees lay eggs singly in cells of the honeycomb. After the eggs hatch, worker bees feed the larvae in the cells and cap them when the larvae pupate. A drone is pictured emerging from a cell.

PHOTOS BY KATHY KEATLEY GARVEY, UC, DAVIS.

The three major considerations in tree fruit pollination are: 1) compatibility, ­2) pollination, and 3) bees as pollinators. To increase the likelihood of producing an acceptable commercial crop, you should consider each topic thoroughly, before advancing to the next topic.

Compatibility. This refers to the ability of a viable pollen grain to grow a pollen tube down the style and complete fertilization in the ovule. Many varieties of many species of fruits are self-incompatible; the style contains enzymes that destroy pollen tubes originating from the pollen from the same flowers or flowers of the same variety. The compatibility of fruit-tree pollination ranges from self-incompatible, progresses through various levels of partial compatibility, and ends with complete compatibility. Be sure to check with nursery personnel or other credible sources, to be sure that compatible pollen will be moved around the orchard.

Pollination. This is simply the transfer of viable pollen from the anthers to a receptive stigma of a flower. While in a few self-pollinating plants, pollen reaches the stigma before the flower opens, most flowers have various schemes to prevent being pollinated by their own pollen. Wind and water can move pollen around from a number of types of plants, but most fruit-tree pollen is heavy and relatively sticky, requiring vectors (pollinators) to move it.

Bees. The most efficient pollinators tend to be bees. Bees generally rely on plant nectar as a carbohydrate source, and pollens for the rest of their nutritional requirements.

Solitary bees, such as leafcutting bees, include single, mated females that find or build a nesting site, then collect nectar and pollen to provision their brood (larvae and pupae). The bee combines the nectar and pollen into a “dough ball” of paste-like consistency and lays an egg on the food. She seals the cell and repeats this process many times. After a larva consumes the food, it will pass through the stages of larva, pupa, or adult. Then, next spring the bees emerge, and mate, and the cycles are repeated.

Social bees, like bumblebees and honeybees, live in colonies with specific individuals performing various duties, often depending upon age, to keep the colonies functional. Like other bees,  bumblebees and honeybees are heavily covered with hairs so that, as they visit the flowers to obtain their food, they wipe the pollen over the reproductive parts of the flowers. Bumblebees are generalists that may visit many different species of plants on a foraging trip. Honeybees tend to be much more species-specific and confine their foraging trips to a single species. In both cases, the food is returned to the colonies and fed to all inhabitants.

Growers rely primarily on honeybees for commercial crop pollination, due to their crop constancy and the ability to saturate a crop with huge numbers of bees. Many growers in the eastern states rely on mason bees for apple pollination, but honeybees usually are in high demand. ­Pollination is a numbers game. There must be enough bees to move the pollen throughout the orchards. If ­honeybee colonies are of adequate strength (in frames of bees), most crops can be pollinated adequately with one colony per acre. With smaller orchards (40 acres or fewer), peripheral placement of colonies should be adequate. With larger orchards, some of the hives should be distributed throughout the orchard as well as around the periphery.

Honeybees foraging for nectar are apt to begin in the morning and continue foraging throughout the day, if nectar is constantly produced. Pollen foragers will begin collecting pollen when the moisture conditions are appropriate. This may be a bit later in the morning. Usually, pollens are liberated (dehisced) from the anthers early in the day. Often, bees can remove all the pollen in a few hours. Thus, pollen foragers will be in the flowers only until the pollen has been removed for the day.

Pesticides

Pesticides of various types, especially fungicides, are likely to be used during bloom to protect the flowers from diseases that may enter and blast the flowers, ruin the fruit, or spread diseases into the branches. Even if you carefully read the precautions on fungicide labels, you will not know if the fungicides might be harmful to bees. This is because the registration process requires only a 24- or 48-hour mortality test on adult honeybees. Fungicides do not kill adult honeybees quickly. In fact, adult bees probably will not be negatively affected by fungicides at all. However, when the fungicides carried on incoming pollens get mixed into the food chain of the colonies, they can have detrimental impacts on the brood. Often, depending upon concentrations fed to the larvae, the larvae may simply die, or the larvae may continue to feed and grow. Problems can occur when the pupae try to chew out of their cells and become functional adults. Again, depending upon the dose, new adult bees can fail to emerge completely. They stick their heads out of the cells and extend their mouthparts as if soliciting for food. Some emerge with ­distorted body parts. They will be useless to the colony and will not live very long.

The best way to protect pollinators from negative effects of exposure to pesticides is to refrain from spraying any pesticide on the pollen-laden flowers or the pollen-collecting bees while the bees are foraging, which usually occurs early in the day. If you visit the orchards and look for pollen foraging, you can determine the safest time for spraying.